Various sensor devices require a heater. One group of such devices is formed by flow sensors, where changes in the thermal distribution around the heater are used to measure a fluid flow. Such a device is e.g. disclosed in US 2007/0241093. Another group of such devices comprises gas sensors, where the heater is used to heat a sensing material, such as a metal oxide, to an operating temperature. A device using such a hot plate is e.g. disclosed in WO 95/19563.
This type of devices can be integrated onto a semiconductor substrate, in which case, the heater is advantageously arranged on a membrane over an opening in the semiconductor substrate, thereby reducing the thermal loss as compared to devices where the heater is arranged over the bulk of the substrate material. Arranging the heater on a membrane has a series of advantages, such as reducing power consumption, increasing sensitivity and reducing the time required for switching on the device.